Vehicles equipped with a friction clutch for engaging/disengaging the transmission of an engine drive force and a device for operating the friction clutch are well known. An example of a device used to operate a clutch is a clutch lever of a motorcycle.
Generally, as the size of a vehicle becomes larger, the capacity of the friction clutch becomes larger, and the force necessary to disengage the friction clutch also becomes larger. This, however, increases the load required to operate the clutch, which in turn impacts the operational burden on the rider. In order to reduce the load required to operate the clutch, and in turn the operational burden for the rider, a technique for attaching a so-called “power-assist device” to a friction clutch has been suggested, for example, in reference to Japanese Patent Application Publication 52-004955 and Japanese Patent 3381442.
The power-assisted friction clutch disclosed in Japanese Patent Application Publication 52-004955 includes a drive force transmission side rotating body having a drive force transmission side friction member, a driven force transmission side rotating body having a driven force transmission side friction member, and a pressure contact member for receiving the biasing force of a pressure contact spring to press the drive force transmission side friction member and the driven force transmission side friction member into contact with each other. As a power-assist device, the friction clutch includes an operating side rotating body which rotates integrally with the driven force transmission side rotating body, an operated side rotating body which rotates opposite to the operating side rotating body through a friction member, and a ball cam for moving the pressure contact member in a direction which separates the drive force transmission side friction member and the driven force transmission side friction member from each other when the operated side rotating body is under torque conditions.
The friction clutch, when disengaged, presses the operating side rotating body, which is pressed against the operated side rotating body through the friction member, transmitting torque to the operated side rotating body via the operating side rotating body. A ball cam moves the pressure contact member in a direction which separates the drive force transmission side friction member and the driven force transmission side friction member from each other. As a result, the friction clutch uses part of the torque of the driven force transmission side rotating body as force which separates the drive force transmission side friction member and the driven force transmission side friction member from each other. Therefore, the force necessary to separate the drive force transmission side friction member from the driven force transmission side friction member, i.e. the force necessary for disengaging a clutch, can be decreased.
However, the friction clutch has a problem in that a shift shock is relatively large when the clutch is engaged in an idling state like shifting gears from a neutral to a first gear. That is, since the friction clutch has a power-assist device, the force necessary for disengaging the clutch decreases, but when the clutch is engaged, since the drive force transmission side friction member, which is rotating, and the driven force transmission side friction member are pressed into contact with each other by the large capacity of the pressing contact spring, a relatively large shock occurs in engaging the clutch. Therefore, a problem arises in regard to the quality of ride, or feel, sensed by a rider of the vehicle, which tends to deteriorate with this known friction clutch setup.